JavaScript is one of the most popular programming languages in the world and a skill that is in high demand in the job market. Having a solid understanding of JavaScript is crucial for anyone looking to pursue a career in web development or programming.
In this comprehensive guide, we will provide sample answers to some of the most common JavaScript interview questions asked by hiring managers and recruiters. The questions cover basics of JavaScript as well as advanced concepts like promises, closures, prototypal inheritance and more.
Whether you are preparing for an upcoming JavaScript interview or just want to sharpen your skills – this guide will help take your JavaScript knowledge to the next level. Let’s get started!
Basic JavaScript Interview Questions
Below are some common interview questions on core JavaScript fundamentals:
What is JavaScript?
JavaScript is a lightweight, interpreted programming language that is used to build dynamic, interactive web applications. Some key highlights:
- High-level dynamic scripting language for web pages and applications
- Runs on the client-side so executes in the browser rather than on the server
- Can be used to manipulate DOM and website content on the fly
- Object-oriented language with first-class functions
- Complementary to and often used with HTML and CSS
- Javascript code is embedded in HTML documents and interpreted by the browser
Although JavaScript is primarily used in the browser, it can also be run on the server side using runtime environments like Node.js.
What are the differences between JavaScript and Java?
- JavaScript is a scripting language while Java is a compiled programming language.
- JavaScript is dynamically typed while Java is statically typed.
- JavaScript runs in browsers and is used for client-side development. Java runs on JVMs (Java Virtual Machines) and is used more for server-side development.
- JavaScript is interpreted. Java is compiled to bytecode which runs on JVMs.
- JavaScript is single-threaded while Java supports multithreading.
- JavaScript has prototype-based inheritance whereas Java uses class-based inheritance.
Explain event handling in JavaScript.
Event handling refers to the ability to execute code in response to user inputs and system events.
Some common ways to handle events in JavaScript include:
- HTML event attributes like onclick, onsubmit etc.
- DOM event handlers like addEventListener, attachEvent to attach event listeners to elements
- Setting event handler properties directly on DOM elements like element.onclick = function(){…}
For example:
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// HTML event attribute
<button onclick=”alert(‘Hello World’)”>Click Me</button>
// addEventListener method
var button = document.querySelector(‘button’);
button.addEventListener(‘click’, function(){
alert(‘Hello World’);
})
// Setting onclick property
button.onclick = function(){
alert(‘Hello World’);
}
This allows JavaScript to respond dynamically to user interactions like clicks, key presses, mouse movements etc.
What are variables in JavaScript?
Variables are used to store data values so that they can be referenced and used later in the code.
In JavaScript, variables can be declared using var, let or const:
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// Declare variable
var age = 30;
// Declare constant
const PI = 3.14;
- var – Function scoped
- let – Block scoped
- const – Block scoped and cannot be reassigned
Key notes:
- JavaScript is dynamically typed, so no need to specify type
- Can store all types of values – strings, numbers, booleans etc.
- Variables declared with const cannot be reassigned
- Try to use const by default and only use let if the value needs to change
What are the differences between var, let, and const?
The differences between the three variable declarations in JavaScript are:
- var declarations are function scoped while let and const are block scoped.
- var variables can be redeclared and reassigned within a function.
- let variables can be reassigned but not redeclared.
- const variables can neither be redeclared nor reassigned.
- Function parameters are always var by default.
- let and const are not hoisted like var.
- Prefer using const by default unless you need to mutate the value later.
Intermediate JavaScript Questions
Let’s move on to some slightly tougher JavaScript interview questions and answers:
What are closures in JavaScript?
A closure is the combination of a function bundled together with references to its surrounding state. In JavaScript, closures are created whenever a variable that is defined outside the function is accessed from within the function.
Some key characteristics of closures:
- Closures have access to the outer function’s scope, even after the outer function returns.
- They store references to the outer function’s variables and parameters.
- The closure maintains a connection to the outer function which keeps the variables in memory.
- Useful for encapsulating logic and data within a unit without exposing details.
Here is a simple example:
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function outer() {
var count = 0; // local variable
function inner() {
count++; // access a variable from the outer scope
return count;
}
return inner;
}
var closure = outer();
closure(); // returns 1
closure(); // returns 2
Even after outer() has returned, the reference to count persists via the closure. The data is private and protected between function calls.
Explain how this keyword works in JavaScript.
The this keyword references the object that the function is executing in. Some key points:
- In simple functions, this refers to the global object which is window in browsers.
- In object methods, this refers to the parent object.
- The value of this depends on how the function is called.
- Can explicitly set context using call(), apply(), bind().
- In strict mode, this is undefined rather than the global object.
- Arrow functions preserve the this binding of the enclosing scope.
For example:
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// In global context, this = window
this === window; // true
function myFunc() {
// In function context, this = window
this === window; // true
}
const person = {
name: ‘John’,
present: function() {
// In method, this = parent object
console.log(`Hello, my name is ${this.name}`);
}
}
person.present(); // Hello, my name is John
The value of this is determined based on how the function is called.
What is prototypal inheritance in JavaScript?
All JavaScript objects have a prototype property that allows them to inherit properties and methods from other objects. This is known as prototypal inheritance and is core concept in JavaScript.
- The properties and methods defined on the prototype are inherited by all instances.
- The __proto__ property points to the object’s prototype.
- Can chain prototypes together to share properties up the chain.
- Prototypal inheritance is more flexible than classical inheritance.
For example:
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// Constructor function
function Person(name) {
this.name = name;
}
// Prototype method
Person.prototype.introduce = function() {
console.log(`Hi, I’m ${this.name}`);
}
// Inherits from Person
function Student(name, major) {
Person.call(this, name);
this.major = major;
}
// Inherit methods from Person
Student.prototype = Object.create(Person.prototype);
// Student inherits introduce()
const s1 = new Student(‘John’, ‘Computer Science’);
s1.introduce(); // Hi, I’m John
Prototypal inheritance allows objects to reuse logic across linked prototypes.
What is the difference between == and === operators?
- == checks for value equality only after type coercion.
- === checks for value equality without type coercion.
This code illustrates the difference:
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// Loose equality with coercion
0 == ”; // true
1 == ‘1’; // true
// Strict equality without coercion
0 === ”; // false
1 === ‘1’; // false
Some key differences:
- === is faster as it avoids coercion.
- == can cause unexpected results due to forcing types to match.
- === prevents unintended type converting bugs.
- Use === unless you specifically need to allow type conversion.
Advanced JavaScript Questions
Let’s now look at some advanced JS interview questions on topics like asynchronous programming